Abstract The 90 kD heat shock proteins (Hsp90) are molecular chaperones that are responsible for the folding of select proteins, many of which are directly associated with cancer progression. Consequently, inhibition of the Hsp90 protein folding machinery results in a combinatorial attack on numerous oncogenic pathways. Seventeen small molecule inhibitors of Hsp90 have entered clinical trials for the treatment of cancer, all of which bind the Hsp90 N-terminus and exhibit pan-inhibitory activity against all four Hsp90 isoforms. Pan-Inhibition of Hsp90 appears to be detrimental as toxicities have been reported alongside induction of the pro-survival heat shock response. The development of Hsp90 isoform-selective inhibitors represents an alternative approach towards the treatment of cancer and may reduce side effects observed with pan-Hsp90 inhibition. This application proposes optimization of a novel class of Hsp90?-selective inhibitors that induces the degradation of select Hsp90 clients without simultaneous induction of Hsp90 levels. Preliminary work involving Hsp90?-selective compounds at University of Notre Dame has established that Hsp90? inhibition offers a targeted and safer therapeutic approach for the treatment of cancer. In this Phase I, PrevAllergy (d/b/a Grannus Therapeutics) will optimize the efficacy and drug like properties of the novel Hsp90?-selective inhibitor using a rational, structure- based drug design approach to obtain a lead compound. Aim 1. Optimize the current lead Hsp90?-selective compounds by synthesizing rationally designed new analogs to improve affinity, pharmacokinetic properties and metabolic stability of the discovered series of Hsp90?-selective inhibitors. The working hypothesis is that introduction of functional groups will increase drug-likeness of our current lead compounds. Aim 2. Evaluate Hsp90?-selective inhibitors developed in Aim 1 utilizing pharmacokinetic assays and in vitro cellular studies, and in vivo studies to assist in the identification of a drug candidate. Based on the team's preliminary cellular studies involving multiple cancer cell lines, cancers with increased dependency upon Hsp90? have been identified, animal models of these identified cancer will be treated. Upon completion of the proposed work, Grannus Therapeutics will have identified lead compounds with improved drug-like properties to progress into Investigational New Drug (IND)-enabling studies. The result from Phase II will lead to an IND application and subsequent initiation of clinical trials to treat the identified cancers that still represent an unmet medical need. !